Non-linear resistor of sintered zinc oxide

ABSTRACT

A NON-LINEAR RESISTOR. THE RESITOR HAS A SINTERED DISC OF ZINC OXIDE AND TWO ELECTRODES, ONE APPLIED TO EACH OF THE OPPOSITE SURFACES OF SAID SINTERED DISC. ONE OF SAID TWO ELCTRODES IN A SILVER ELECTRODE IN NON-OHMIC CONTACT WITH ONE OF SAID OPPOSITE SURFACES, AD THE OTHER OF SAID TWO ELECTRODES IN OHMIC CONTACT WITH ANOTHER OF SAID OPPOSITE SURFACES.

March 9, 1971 TAKESHI MASUYAMA' ET AL ,5

NON-LINEAR RESISTOR OF SINTERED ZINC OXIDE Filed'April 24, 1968INVENTORS TAKESHI MASUYAMA MIC HIO MATSUOKA ATTORNEYS United StatesPatent 3,570,002 NON-LINEAR RESISTOR 0F SINTERED ZINC OXIDE TakeshiMasuyama and Michio Matsuoka, Osaka, Japan, asslgnors to MatsushitaElectric Industrial (10., Ltd., Osaka, Japan Filed Apr. 24, 1968, Ser.No. 723,746

Claims priority, application Japan, Apr. 26, 1967,

4207,34: Int. Cl. H01l 3/16 US. Cl. 317238 9 Claims ABSTRACT OF THEDISCLOSURE A non-linear resistor. The resistor has a sintered disc ofzinc oxide and two electrodes, one applied to each of the oppositesurfaces of said sintered disc. One of said two electrodes is a silverelectrode in non-ohmic contact with one of said opposite surfaces, andthe other of said two electrodes in ohmic contact with another of saidopposite surfaces.

where V is the voltage across the resistor, I is the current flowingthrough the resistor, C is a constant corresponding to the voltage at agiven current, and the exponent n is a numerical value greater than 1.The value of n is calculated by the following equation:

mi 2W1) where V and V are the voltages at given currents I and Irespectively. Conveniently, I and I are 10 ma. and 100 ma.,respectively. The desired value of C depends upon the use to which theresistor is to be put. It is ordinarily desirable that the value of n beas large as possible since this exponent determines the degree to whichthe resistors depart from ohmic characteristics.

Silicon carbide varistors are most widely used as nonlinear resistorsand are manufactured by mixing fine particles of silicon carbide withwater, ceramic binder and/ or conductive material such as graphite ormetal powder, pressing the mixture in a mold to the desired shape, andthen drying and firing the pressed body in a non-oxidizing atmosphere.Silicon carbide varistors with conductive materials are characterized bya low electric resistance, i.e. a low value of C and a low value of n,whereas silicon carbide varistors without conductive materials have ahigh electric resistance, i.e. a high value of C and a high value of n.It has been difficult to manufacture silicon carbide varistorscharacterized by a high n and a low C. For example, silicon carbidevaristors with graphite have been known to exhibit n-values from 2.5 to3.3 and C-values from 6 to 13 at a given current of 100 ma., and siliconcarbide varistors without graphite have n-values from 4 to 7 andC-values from 30 to 800 at a given current 3,570,002 Patented Mar. 9,1971 of 1 ma. for a given size of varistor, e.g. 30 mm. in diameter and1 mm. in thickness.

Conventional rectifiers comprising selenium or cuprous oxide have ann-value less than 3 and a C-value of 5 to 10 at a given current of ma.for a specimen size of 20 mm. in diameter. In this case, the thicknessof the sample does not affect the C-value.

A germanium or silicon p-n junction resistor has an extremely high valueof it but its C-value is constant, e.g. on the order of 0.3 or 0.7 at agiven current of 100 ma., because its diffusion voltage for the VIcharacteristics is constant and can not be changed greatly. It isnecessary for obtaining a desirable C-value to combine several diodes inseries and/or in parallel. Another disadvantage of such diodes is thecomplicated steps involved in their manufacture, with resultant highcost. As a practical matter, the use of diode resistors is notwidespread at the present in view of their high cost even though theyhave a high value of n.

An object of this invention is to provide a non-linear resistor having ahigh value of n and a low value of C.

A further object of this invention is to provide a nonlinear resistorcapable of being made by a simple manufacturing method and hence at alow cost.

A further object of this invention is to provide a nonlinear resistorcharacterized by a high stability with respect to temperature, humidityand electric load.

Another object of this invention is to provide a nonlinear resistor, theC-value of which can be controlled.

These objects are accomplished by providing a nonlinear resistorcomprising a sintered disc of zinc oxide and two electrodes, one appliedto each of the opposite surfaces of said sintered disc, one of said twoelectrodes being a silver electrode in non-ohmic contact with one ofsaid opposite surfaces, and the other of said two electrodes being inohmic contact with another of said opposite surfaces.

These and other objects of the invention will become apparent uponconsideration of the following description taken together with theaccompanying drawing, in which the single figure is a partlycross-sectional view through a non-linear resistor in accordance withthe invention.

Before proceeding with a detailed description of the non-linearresistors contemplated by the invention, their construction will bedescribed with reference to the aforesaid figure of the drawing, whereinreference character 10 designates, as a whole, a non-linear resistorhaving, as its active element, a sintered wafer 1 of electricallyconductive ceramic material according to the present invention.

Sintered wafer 1 is prepared in a manner hereinafter set forth, and isprovided with a pair of electrodes 2 and 3 having specified compositionsand applied in a suitable manner, hereinafter set forth, on two oppositesurfaces of the wafer.

One of said pair of electrodes, for example, the electrode 2 is a silverelectrode in non-ohmic contact with the water 1, and the other electrode3 is in ohmic contact with the wafer 1 in accordance with the invention.

The wafer 1 is a sintered plate having any one of a number of variousshapes such as circular, square, rectangular, etc. Wire leads 5 and 6are attached conductively to the electrodes 2 and 3, respectively, by aconnection means 4, such as solder or the like.

According to the present invention, sintered water 1 consistsessentially of, as an active ingredient, zinc oxide (ZnO). It ispreferable that said zinc oxide have incorporated therein a proportionof 0.05 to 10.0 mol percent of at least one oxide selected from thegroup consisting of aluminum oxide (A1 0 iron oxide (Pesos), bismuthoxide (Bi O magnesium oxide (MgO), calcium 4 oxide (CaO), nickel oxide(NiO), cobalt oxide (C), Composition of silver electrode (wt. percent):niobium oxide (-Nb O tantalum oxide (Ta O zir- PbO1.2-l7 conium oxide(ZrO tungsten oxide (W0 cadmium SiO 0.1-6.0 oxide (CdO), and chromiumoxide (Cr O 2 a It has been discovered according to the invention that rz :i sintered wafer 1 will be in non-ohmic contact with said 0 CdO-0*2.0silver electrode 2 when said silver electrode is prepared (3110-0-40applying Silver Paint to one Surface of said Wafel: and Since thenon-linearity of the novel resistors can be i g F thus coated Wafer at1,000 to m an attributed to the non-ohmic contact between said sinteredoxldlzlng atmosphere such as alt f oxygen- 10 wafer 1 and said silverelectrode 2, it is necessary for Table 1 Shows operable f optlmalFomPosltlons of obtaining a desirable C-value and n-value to control thesllver electrode 2 after beam? for Curmg m compositions of the sinteredwafer 1 and the silver elecproduce the novel non-linear resistors inaccordance with node 2 the invention. It is necessary for achieving alowvalue of C for the Important for eehlevmg a non-01111110 Contact thatresultant non-linear resistors that the sintered wafer have Sald sllvel'electrode be lf p hl f at a p an electrical resistivity less than 10ohm-cm, said electure of 100 to 111 all OXldlZlng atmosphere Such tricalresistivity being measured by a four point method as air and oxygen, asilver paint which is applied to the in a per Se conventional surface ofsaid sintered wafer and which has a composi- Th i d b d 1 can b preparedb a per Se w ll tion similar With respect to the SOlid ingredients tothat known ceramic technique, The starting materials having 0f the firedSilver electrode the compositions according to the invention are mixedThe other electrode, in Ohmic Contact With the Wafer, in a Wet mill soas to produce homogeneous mixtures. is formed y Vacuum-evaporating ametal Selected from The mixtures are dried and pressed in a mold intodesired the g p consisting of aluminum, zine. tin and i i shapes at apressure of from 100 kg./cm. to 1000 kg./ in accordance With t einventioncm. The pressed bodies are sintered in air at 1250 C. A emethod fo making Said electrode Which s to 1450 C. for l to 3 hours, andthen furnace cooled in ohmic contact according to the invention is toplate to room temperature (from about 15 to about C.).electro-chemically a metal selected from the group con- The pressedbodies are preferably sintered in a nonsisting of silver, copper,nickel, zinc, and tin. oxidizing atmosphere such as nitrogen or argonwhen it is A further operable electrode which is in ohmic con- 30desired to reduce the electrical resistivity. The electrical tact can beformed by a spray metalized film of a metal resistivity also can bereduced by air quenching from the selected from the group consisting ofcopper, tin, zinc sintering temperature to room temperature even whenand aluminum. the pressed bodies are fired in air.

TABLE I Operable composi- Optimal composition of electrode (wt. percent)tion of electrode wt. percent (Ag; Ag PbO SiO2 B203 Biz03 CdO CuO It hasbeen discovered according to the invention that The mixtures can, ifdesired, be preliminarily calcined a sintered wafer of zinc oxide of theaforesaid composiat 700 to 1000 C. and pulverized for easy fabricationtion has an excellent non-linearity as a non-linear resistor in thesubsequent pressing step. The mixture to be pressed and a high stabilitywith respect to humidity, temperacan be admixed with a suitable bindersuch as water, ture, and electric load when it is provided, at onesurface polyvinyl alcohol, etc.

thereof, with a silver electrode which is in non-ohmic con- The sinteredbodies in wafer form are coated on one tact therewith and has theaforesaid composition, and at surface thereof by a silver electrodepaint in a per se the other surface thereof, with another electrode inohmic conventional manner such as by a p y method, Screen contacttherewith, as described hereinbefore. Such a nonprinting method orbrushing method. It is necessary that linear resistor is anon-symmetrical type and can be conthe silver electrode paint have asolid ingredient comnected at said silver electrode to a positiveterminal of P ition as defined in Tables 1 and 2 after it is fired at aDC source and, at said electrode in ohmic contact, to 100 C. to 850 C.in air- Solid ing having coma negative terminal of DC source. A greatfeature of PositionS defined in Tables 1 and 2 can be P p in thenon-linear resistor of the non-symmetrical type aca P Se Conventionalmanner y mixing commercially cording to the invention is the combinationof a high available powders with organic resin such as p y vinyl n-valueand a low C-value, lower than 1.0 at a given and Phenol resin in aOrganic solvent h as hutyl current of 100 tate, toluene or the like soas to produce silver electrode Table 2 shows optimal compositions forthe combinap tion of the sintered wafer 1 and the silver electrode in Tsllver Powder y be in the form of metallic Silver, accordance with theinvention. A non-linear resistor with P 111 the form of Silver carbonateSilver OXide, or such optimal compositions has a C-value lower than 0.8y otherform which during firinget h temperatures at a given current of100 ma., an n-value higher than employed i be Converted to rrletalhe Y eq s 0 and a high stability with respect temperature, the term sllver asused throughout this specification humidity and electric load, when itis provided with the e the ms pp he in c i n w h h other electrode 3consisting of any one of the aforesaid silver florrlpesl'flefl befere 1tfired; 1S meant to include operable electrodes in ohmic contacttherewith in accordsllvel' 111 e 3 f which d1lr1I 1g firlrlg W111 beconverted ance with the invention. to metallic silver. The viscosity ofthe resultant silver electrode paints can be controlled by the amountsof resin TABLE Z-QOP'DIMAL COMPOSITION OF SINTERED and solvent. Theparticle sizes of the solid ingredients AND SILVER ELECTRODE also arerequired to be controlled so as to be in the range Composition ofsintered wafer (mol percent): of 0.1 to 5 ZnO-100-98 A sintered wafer,after being provided on one surface 'ZFe O 02.0 thereof with said silverelectrode, is provided, on the other surface thereof, with an electrodein ohmic contact by vacuum evaporation, electrochemical plating or spraymetallizing in a per se well known manner.

Lead Wires can be applied to the silver electrode and the electrode inohmic contact in a per se conventional manner by using conventionalsolder having a low melting point. It is convenient to employ aconductive adhesive comprising silver powder and resin in an organicsolvent for connecting the lead wires to the silver electrode and theohmic contact electrode.

Non-linear resistors according to this invention have a high stabilitywith respect to temperature and in a load life test, which is carriedout at 70 C. at rated power for 500 hours. The n-value and C-value donot change significantly after being subjected to heating cycles and aload life test. It is preferable for achieving a high stability withrespect to humidity that the resultant non-linear resistors be embeddedin a humidity proof resin such as epoxy resin and phenol resin in a perse well known manner.

According to the invention, it has been discovered that the curingmethod for the applied silver electrode paint has a great effect on then-value of the resultant nonlinear resistors. The n-value will not beoptimal when the applied silver electrode paint is heated in anon-oxidizing atmosphere such as nitrogen or hydrogen for curing. It isnecessary for obtaining a high n-value that the applied silver electrodepaint be cured by heating in an oxidizing atmosphere such as air oroxygen.

A silver electrode prepared by any method other than by silver paintingresults in a resistor having a poor n-value. For example, the sinteredbody does not act as a non-linear resistor when it is provided with asilver electrode by electroless plating or electrolytic plating in aconventional manner. A silver electrode prepared by vacuum evaporationor chemical deposition results in a resistor having an n-value less than3.

The following examples are given as illustrative of thepresently-preferred method of proceeding according to the presentinvention; however, it is not intended that the scope of said inventionbe limited to the specific examples.

Starting materials according to Table 3 were mixed in a wet mill for 5hours.

The mixtures were dried and pressed in molds into discs of 13 mm.diameter and 2.0 mm. thick at a pressure of 340 kg./cm.

The pressed bodies were sintered in air at 1350 C. for 1 hour, and thenquenched to room temperature (about 15 to about 30 C.). The resultingsintered discs were 10 mm. in diameter and 1.5 mm. thick. The sintereddiscs were coated on one surface thereof with a silver electrode paintby a conventional brushing method. The silver electrode paint employedhad the solid ingredient composition according to Table 4 and wasprepared by mixing with vinyl resin in amyl acetate. The coated discswere fired at 500 C. for 30 minutes in air. The other surfaces wereprovided with a spray metalized film of aluminum by a per se well knowntechnique.

Lead wires were attached to the silver electrodes and the aluminumelectrodes by means of conductive silver paint. The electriccharacteristics of the resultant resistors measured in that direction ofeasy current flow are shown in Table 3.

TABLE 4 Composition of silver electrode wt. percent:

Ag 9O PbO 7.0 SiO 2.0 B203 CuO 0.3

EXAMPLE 2 Sintered discs each having a composition of 99.5 mol. percentof zinc oxide and 0.5 mol. percent of iron oxide were prepared in thesame manner as in Example 1. The sintered discs were 10 mm. in diameterand 1.5 mm. thick. Various silver electrode paints were applied to onesurface of the respective sintered discs and the coated discs were firedat 500 C. for 30 minutes in air. The silver electrode paints had solidingredient compositions as shown in Table 5 and are prepared by mixing100 parts by weight of said solid ingredient compositions with 1 to 20parts by Weight of epoxy resin in 20 to 40 parts by weight of butylalcohol. The other surfaces were provided with a nickel electrode by anelectroless plating method. The resultant non-linear resistors haddesirable C-values and n-values as indicated in Table 5. It will thus bereadily understood that the electrode compositions have a great effecton the electrical characteristics of the resultant nonlinear resistors.

EXAMPLE 3 The resistors of Example 1 were tested according to themethods used for testing electronic component parts. The load life testwas carried out at 70 C. ambient temperature at a 1 watt rating powerfor 500 hours. The heating cycle test was carried out by repeating 5times a cycle in which said resistors were kept at C. ambienttemperature for 30 minutes, cooled rapidly to -20 C. and then kept atsuch temperature for 30 minutes. After the heating cycle and load lifetests, the C-value and n-values did not change more than 1.5% and 3%,respectively.

1 At a given current of 100 ma.

What is claimed is:

1. A non-linear resistor comprising a sintered disc of zinc oxide andtwo electrodes, one applied to each of the opposite surfaces of saidsintered disc, one of said two electrodes being a silver electrode innon-ohmic contact with one of said opposite surfaces, and the other ofsaid two electrodes being in ohmic contact with another of said oppositesurfaces.

2. A non-linear resistor according to claim 1, wherein said sintereddisc consists essentially of zinc oxide.

3. A non-linear resistor according to claim 1, wherein said sintereddisc consists essentially of 99.95 to mol percent of zinc oxide and 0.05to 10.0 mol percent of at least one oxide selected from the groupconsisting of F6203, A1203, Bi203, CaO, COO, Nb205, T3205, ZrO W03, andCr O 4. A non-linear resistor according to claim 1, wherein said silverelectrode is percent by weight Ag.

5. A non-linear resistor according to claim 1, wherein said silverelectrode consists essentially of 0.25 to 27 percent by weight PbO, 0.02to 15 percent by weight SiO 0.01 to 15 percent by weight B 0 0 to 6.0percent by weight Bi O 0 to 6.0 percent by weight CdO, 0 to 6.0 percentby weight CuO and the balance silver.

6. A non-linear resistor according to claim 1, wherein said sintereddisc consists essentially of 100 to 98.0 mol percent ZnO and 0 to 2.0mol percent R 0 and said silver electrode consists essentially of 1.2 to17.0 percent by Weight PbO, 0.1 to 6.0 percent by weight SiO 0.06 to 6.0percent by weight B 0 '0 to 2.0 percent by weight Bi O 0 to 2.0 percentby weight CdO, 0 to 2.0 percent by weight CuO and the balance silver.

7. A non-linear resistor according to claim 1, wherein said otherelectrode in ohmic contact consists essentially of an evaporated film ofa metal selected from the group consisting of aluminum, zinc, tin andindium.

8. A non-linear resistor according to claim 1, wherein said otherelectrode in ohmic contact consists essentially of an electrochemicallyplated film of a metal selected 8 from the group consisting of silver,copper, nickel, zinc and tin.

9. A non-linear resistor according to claim 1, wherein said otherelectrode in an ohmic contact consists essentially of a spray metalizedfilm of a metal selected from the group consisting of copper, tin, zincand aluminum.

References Cited JAMES D. KALLAM, Primary Examiner 0 US. Cl. X.R.

